Interpretive Summary: High litter size in the Chinese Meishan sow is well documented although mechanisms responsible for improved litter size are poorly defined in swine. Uterine development in white crossbred and Chinese Meishan neonatal gilts was compared to identify uterine development factors that may influence adult uterine function and litter size. Crossbred and Meishan gilts were necropsied on Day 1, 14, 28, 42, or 56 of age and the ovaries and uterus were weighed. The uterus was examined histologically and cultured in vitro to determine uterine protein secretion. Unique uterine secreted proteins were identified (Day 1 to 42) during initial uterine development and endometrial glandular development. Ovarian follicle activity was detected after Day 42 in Meishan gilts and this resulted in marked increases in ovarian and uterine weights as well as uterine histological changes in this breed. Although specific regulatory roles for rlocally produced uterine proteins remain to be defined, the breed differences and the increase in specific uterine proteins during the ovary- independent stage suggest that they may influence uterine development and therefore affect adult uterine capacity.

Technical Abstract:
Thirty-five crossbred (n=7/age group) and 22 Meishan (n=4 or 5/age group) gilts were necropsied on Day 1, 14, 28, 42, or 56 of age (birth = Day 0) to identify events related to uterine wall development that may influence adult uterine function and litter size. At necropsy, a cross-section of one uterine horn was fixed for histomorphometric study and minced uterine tissue (135 or 200 mg) was cultured with 50 uCi 3H-leucine for 24 h. Secreted proteins were identified by two-dimensional PAGE, fluorography, and incorporation of radioactivity. Body weights at necropsy were similar for both breeds and increased (p<.01) between Day 1 and 56. Ovarian weights were similar at birth and increased minimally in both breeds from Day 1 to 42 before increasing (p<.01) in Meishan gilts on Day 56. Likewise, uterine weights were similar at birth for both breeds, increased steadily from Day 1 to 42, and then increased markedly (p<.01) in Meishan gilts on Day 56. Histomorphometric measurements of uterine wall components increased in are as uterine weight increased. Glandular area increased in both breeds by Day 42 but was greater (p<.01) in Meishan than crossbred gilts on Day 56. In gilts of both breeds, secretion of uterine proteins 1, 2a and 2b (doublet), and 3 increased in association with endometrial gland development. A fourth protein was observed in gilts of both breeds but was more abundant in Meishan than in crossbred gilts; a fifth protein was detected in crossbred gilts on Day 56. Although specific regulatory roles for locally produced uterine proteins remain to be defined, the breed differences and the increase in specific uterine proteins during the ovary-independent stage suggest that they may influence uterine development and therefore may affect adult uterine capacity.